Check the Side Switch:
The app utilizes the Ipad's native video player and often the reason sound does not play is because it has not been turned on. Check the side of the iPad and confirm the orange dot is not showing.
Check the Control Center:
Launch the Control Center by swiping up from the bottom of your screen. Look for a bell with a line through it. It will be one of six icons in the middle of the Control Center. If the bell is white, tap on it to turn it black (the off position), then try launching an app with sound to see if your audio has returned
Turn the Sound All the Way Up
In some cases, turning he sound all the way up will correct the issue.